Wavelet analysis ultra-thin metasurface for hypersonic flow control

Metasurfaces are one of the new frontier of material science and engineering, finding widespread applications in a number of fields, including acoustics and aeroacoustics. In the present paper we propose a novel metasurface specifically fabricated for the hypersonic flow transition control, together with a new strategy for metasurface characterization in the ultrasonic regime. Instead of a conventional porous layer, the metasurface here presented consists in a flat plate with a set of regularly distributed sharp slots. We experimentally observed that such a geometry significantly reduces the wall reflection coefficient, which is known to play a fundamental role in the boundary layer transition phenomenon. Numerical simulations led us to interpret the incident wave scattering as the underlying mechanism related to the observed reflection coefficient reduction. The metasurface characterization has been carried out by comparing the conventional reflection coefficient in the Fourier domain with an innovative wavelet transform-based strategy. As an overall result, the surface geometry here proposed has been shown to offer a twofold advantage: i) it is more effective in ultrasonic incident wave control with respect to conventional porous layers, ii) it needs less manufacturing time, a primary requirement of this type of technology. More interesting, we have put in evidence how a multiresolution approach, like that here proposed, can be highly promising as characterization tool for further metasurfaces with a more complex and multi-scale geometry, being wavelets able to capture the multiscale behaviour of the reflected wave, overcoming the well-known limits of Fourier-based strategies of data analysis.